Multiple-characteristic superconductive wire



Nov. 1, 1960 H. o. MoMAHoN 2,958,836

MULTIPLE-CHARACTERISTIC SUPERCONDUCTIVE WIRE Filed July 11, 1957 2sheets-sheet 1 looo LLI .3 soo- Ni soo- 9 `28 C 7 K5 g C c D K6 wINVENTOR.

HOWARD O. McMAHON ATTORNEYS.

Nov. 1, 1960 H. o. MCMA-xoN MULTIPLE-CHARACTERISTIC SUPERCONDUCTIVE WIREFiled July l1, 1957 2 Sheets-Sheet 2 F C. 7 v 44h 44h 44b U U u 44a. u uU42@ dfi? \42a AAA/ 42a 42h 44 42: b,.

F G. aA 7' MQ; l FIG. 5

i Z i l f R21 /:\38

c 36 C K2 c C /-4O f/ t/ F3a INVENTOR. HOWARD O. McMAHON v ATTORNEY;

United States atent MULTIPLE-CHARACTERISTIC SUPERCON- DUCTIVE WIREHoward O. McMahon, Lexington, Mass., assignor to Arthur D. Little, Inc.,Cambridge, Mass.

Filed July 11, 19.57, Ser. No. 671,329

8 Claims. (Cl. 338-32) This invention yrelatesto an improvedconstruction for cryotron circuits. More particularly, it relates to, asan article of manufacture, a-conductor having zones of differentsuperconductive properties formed along its length and to constructionsutilizing conductors of this type to form cryotron circuits. Forexample, such a conductor may be intertwined with a second similarconductor to form a plurality of connected cryotrons.

My improved construction may best be understood from the followingdescription taken in connection with the accompanying drawingsin which:

Figure l is a family of curves for different materials showing how thetemperature at which a material becomes superconductive changes as afunction of applied `magnetic field, the materials being superconductivewhen maintained under the conditions represented by the areas to theleft of and below the respective curves.

Figure 2 is a diagrammatic representation of a cryotron,

Figure 3l is aschematic drawing of a cryotron Hip-flop circuit made fromciyotrons of -the type illustrated in Figure 2,

Figure 4 is a section-through a base conductor having a coatingofdifferent superconductive properties deposited on portions thereof,

,'Figure 5 is asection through the conductor of Figure 4 with thecoating diffused into the conductor to form a composite cryotronconductor made according to my invention,

Figure 6 is a sectional view of another embodiment of a compositeycryotron conductor made accordingttomy invention,

Figure 7 is adiagrammatic representation illustrating how a plurality ofconnected cryotrons may be formed from conductors .of the type`illustrated in Figures 3 and 4, and

Figure 8 .is aschematic diagram of the ,cryotron `iiipflop circuit ofFigure 3 fabricated from a pluralityof cryotrons arranged as in FigureV7.

Similar reference characters refer `to similar parts throughouttheseveral viewsof the drawings.

The cryotron, which lis ya switching element kuseful in digitalcomputers, depends for its operation on the changes vin propertiesofcertainelectrical conductors when subjected to temperaturesapproaching absolutezero. As the temperature approaches absolutezero, inthe absence of a magnetic lield, thesematerials change suddenly from aresistive state to Ka superconductive state invwhich their resistance:is identically zero. vThe temperature at which this change occurs isknown as the transition temperature. Whena magnetic `irield'irsappliedto the conductor, the transitionternperature is lowered, therelationship between applied magnetic held and transition temperaturefor ,anurnber/ofthese materials being shown in Figure l. As showninthisiigure, inthe absencevof a magnetic field tantalum loses allelectrical resistance when reduced to ajtemperature of 4.4 K. or below,lead does so at r17.2 K., and niobium at 8 K.

fi eje In all, there are twenty-one elements, in addition to many alloysand compounds, which undergo transition to the super-conductive state attemperatures ranging between O and 17 K. The presence of a magneticfield causes the transition temperature to move to` a lower value, or,if a constant temperature is maintained, an increase in the magneticfield to a certain level will cause the superconductive material torevert to its normal resistive state. From Figure l it is apparent thata magnetic eld of between 50 and 100 oersteds will cause a tantalum wireheld at 4.2 K. (the boiling point of liquid helium at atmosphericpressure) to change from the superconductive to the resistive state.

The cryotron is a circuit element which makes use of the shift betweenthe superconductive and resistive states of these materials when held atconstant temperatures. For example, Figure 2 illustrates a cryotronhaving a central or gate conductor 2 about which is wound a control coil4, both the gate conductor and the coil being of materials which arenormally super-conductive at depressed temperatures. The ent-ire unit isimmersed in liquid helium to render the gate conductor 2 and the controlconductor 4 superconductive. If a current of suilicient magnitude isapplied to the control conductor, the magnetic eld produced therebycauses the gate conductor Vto transfer from the superconductive to theresistive state. Thus, the control coil and gate wire form anelectrically-operated switch in which the gate can be changed from thesuperconductive to the resistive state by the application of current tothe control coil.

Tantalurn is a desirable material for gate conductors, since itstransition temperature in the SO-to-lGO-oersted region is 4.20 K, theboiling point of helium at a pressure of one atmosphere. Thistemperature is attainable without the use of complicated pressure orvacuum equipment for raising or lowering the temperature of helium.Niobium, which has a relatively high quenching field (the held strengthrequired to render a super-conductive material resistive), is usuallyused as the material for the control coil, since it is desirable, and inmany cases necessary, that the control conductor remain superconductivethroughout the operation of the cryotron, and this coil is subject tosubstantially the same magnetic iields as those imposed on the gateconductor. Additionally, in most applications it is desirable to havethe control conductor in the form Vof a coil such as coil 4 in Figure 2in order to minimize the current necessary to produce a quenching iield.L

in cryotron circuitry, the gate conductor of one cryotron is oftenconnected -in series with the control conductor of another, andtherefore the cryotron must provide a current gain for successfuloperation of the circuit, i.e. the current controlled by the gateconductor of the cryotron should be larger than that required toenergize its control coil. if the control conductor is not in the formof a coil, the current through the conductor required to quench thetantalum gate may produce a held large enough to cause self-quenching ofa tantalum gate connected in series with it. In practice it has beenfound that suitable current gain is obtained in a cryotron having a .G09inch tantalum gate conductor with a single layer control coil of .003inch niobium wire having 250 turns per inch.

A simple bi-stable element, the basic unit of a binary digital computer,may be formed with cryotrons by connecting the gates of two of the umtsin parallel and arranging to have one gate or the other conduct all thecurrent through the combination in the same manner as a vacuum tubeflip-op. Thus, as shown in Figure 3, a cryotron ilip-op may comprise twocryotrons K1 and i2 whose gate conductors Klg and KZg are connectedtogether at one end to a power supply, illustratively shown as a battery6, in series with a limiting resistor R1. Preferably, resistor R1 is ofmuch higher resistance than the flip-flop circuit, so that the powersupply is essentially a constant current source. Gate conductor Klg isconnected in series with control coil KZC, and gate conductor K2g issimilarly connected to control coil K10. The control conductors arereturned to ground to complete the circuit through read-in cryotrons K3and K4, respectively, and read-out cryotrons K5 and K6, respectively, ina manner to be described. When conductor Klg is resistive and conductorIQZg is superconductive, an entirely superconductive path is formedthrough gate conductors KZg and K3g and control conductors K1c and Kc.The path through the series combination including conductor Klg isresistive at this time. Thus, all the current from the power supply willflow through gate conductor K2g and none through conductor Klg, and thissame current flowing through control coil Klc will keep the gateconductor Klg enclosed therein in the resistive state. The flip-flop isthus stable in this position, much as a conventional bi-stable vacuumtube flip-flop is stable in one position or the other. If the availablecurrent from the power supply is less than twice that required toquenchone of the gate conductors, and if a pulse is applied to controlwinding KBC to quench gate conductor K3g to make the path includingconductor KZg resistive, the current divides approximately equallybetween the two paths. There is now insufficient current flow throughcontrol conductor Klc to maintain the gate conductor Klg in itsresistive state. As gate conductor Kg changes from the resistive to thesuperconductive state, a superconductive path is formed through it andcontrol conductor KZC, gate conductor K4g, and control conductor Kc;thus, all the available current ows through this path to quench gateconductor KZg. The flip-flop thus reaches its other stable position.

Similarly, a current pulse of suilcient magnitude, applied to controlconductor K4c, will cause the Hip-flop to revert to the former position.Gate conductor KSg or VKtg will be quenched, depending on whether thesuperconductive path through the flip-flop is through control conductorKSC or Kc, and therefore the conductive states of these gate conductorsare indicative of the position of the Kl-KZ ip-op.

Because of its small size and low cost, the cryotron is an ideal basicswitching element for use in large data processing equipment, which mayutilize many thousands of such elements, connected together in a numberof basic circuits, such as the ip-flop illustrated in Figure 3. Prior tomy invention, the small size of the individual cryotron, in mostrespects an important advantage, has presented a serious problem in thefabrication of the various circuits in which it is employed. The fourterminals of .each cryotron are generally connected to other-superconductive circuit elements by welding, to preservesuperconductivity at the point of connection, and thus cryotron circuitsrequire an average of approximately two welded joints per cryotron, e.g.the flip-flop of Figure 3 has ten internal welded connections, asindicated by the reference characters 7 through 24. The individual gateand control conductors of the cryotrons may be as small as one or twomils in diameter, and, therefore, extreme care must be used in handlingthese elements and forming the welded connections between them, in orderto prevent breakage and to assure correct connection. In fact, thefabrication of cryotron circuits is generally performed under amicroscope and is a time-consuming, tedious job.

Accordingly, it is a principal object of my invention to provide animproved cryotron circuit construction having a minimum of internalconnections, and cryotron gate and control conductors adapted for ecientuse in such construction. It is another object of my invention toprovide a cryotron circuit construction of the above character whoseassembly requires minimum handling of 4 individual cryotrons. It is afurther object of my invention to provide cryotron conductors of theabove character capable of use in mechanized assembly operations. It isyet another object of my invention to provide cryotron conductors of theabove character susceptible of low-cost manufacture. lt is a stillfurther object to provide a cryotron flip-flop using a circuitconstruction of the above character. Other objects of my invention willin part be obvious and in part appear hereinafter.

The invention accordingly comprises an article of manufacture and thefeatures of construction, combina-r tions of elements, and arrangementsof parts which will be exempliiied in the constructions hereinafter setforth, and the scope of the invention will be indicated in the claims.

in general, my invention utilizes composite superconductive wires havingalternate gate and control portions which require different magneticfield strengths to render them resistive at the temperature ofoperation. Thus, a conductor made according to my invention may have agate segment with a relatively low quenching field followed by a controlsegment with a relatively high quenching iield, in turn followed by asecond gate segment, and so on. A conductor having many such segmentsmay be cut according to the number of cryotrons to be made from it andthe manner in which they are to be interconnected.

Accordingly, one composite conductor having two gate and two controlsegments might be substituted for gate Klg, control coil KZC, gate K4g,and control K6c in Figure 3; similarly, a conductor of this type mightbe substituted for the other gate and control conductors. The twocomposite conductors would be connected together at 7 and 24, and thusthe number of internal connections in Figure 3 would be reduced to two,2O percent of the number previously required.

My composite cryotron conductor may comprise a tantalum wire havingniobium-coated portions, with the uncoated portions serving as gatesegments and the coated ones, which have a substantially greaterquenching iield, serving as control segments. Alternatively, a wirehaving a base material with a given superconductive property may haveanother material diiused into it along alternate segments to alter theproperties of those segments so that the treated and untreated portionshave different quenching ield strengths.

In Figure 5 I have illustrated a composite cryotron conductor madeaccording to my invention. As shown therein, a conductor, generallyindicated at 26, comprises ialternate control segments 28 of niobium andgate segments 30 of a niobium-tantalum alloy. The' conductor 26 may bemade, as illust-rated in Figure 4, by iirst forming coatings 32 oftantalum at spaced intervals along a niobium wire 34. The coatings 32may be formed by -any desirable process such as evaporation,electro-deposition, etc. The coated wire is then heated `at a suitabletemperature for a suiiicient period of time to diffuse the tantalumcoatings int-o the niobium base conductor. This results in the compositeconductor 26 of Figure 5, in which the untreated niobium segments 28have control conductor characteristics, i.e. stronger quenching fields,and the segments 30 of the niobium-tantalum ialloy have gatecharacteristics, i.e. weaker quenching iields.

Another composite cryotron conductor embodying the features of myinvention is illustrated in Figure 6. As shown therein, a conductor,generally indicated at 36, comprises a tantalum base conductor havingniobiumcoated control segments 38 and uncoated tantalum gate segments40. The niobium coatings may be formed lby evaporation orelectro-deposition in similar fashion to the coatings 32 of Figure 4.

Conductors 26 and 36 are preferably as small in diameter as possible,the lower size limit being determined mainly by problems in handling.Thus the base conductors V34 and 40 are preferably tive-mil Wires, withthe untreated segments of the finished composite conductors having thesame diameter; the treated segments 30 and 38 may have somewhat greaterdiameters, resulting from the introduction of additional material intothem. The tantalum coatings 32 of Figure 4 should be thick enough toprovide segments 30 having quenching characteristics sufficientlydifferent from those of the niobiurn control segments 28 to providevefficient circuit operation, i.e. the disparity in quenching fields'between the segments 28 and 30 should `be at least 3 'to l. Theniobiumcoatings on the segments 38 should be thick enough to insure completecoverage of the portions of the base conductor beneath.

While the conductors 26 and 36 have been described using niobium andtantalurn, it will be understood that other materials may be provided ifthey have widely different quenching characteristics, to insure that, incircuit operation, the control segments will remain superconductiveduringthe quenching of the gate `segments associated with them. It willalso be understood that composite conductors having segments o-f morethan two quenching vcharacteristics may be provided Within the `purviewof my invention. Thus, a conductor similar to the conductor 36 may beformed with lead-coated segments in addition to the niobium-coated anduntreated tantalum segments. The vtantalum segments would be quenched byrelatively low quenching fields, the leadcoated segments by strongerfields, and the niobiumcoated ones by still stronger fields.

My composite superconductive wire may be used to form a plurality ofconnected cryotrons, as illustrated in Figure 7. As shown therein,composite conductors 42 and 44 have gate and control segments 42a and42b, and 44a and 44b, respectively. The conductors may be of the typeillustrated in Figures 5 or 6, Iand they yare preferably wound bysuitable machinery to form a stock material having any desirable numberof sections, each section being an individual cryotron, with a controlsegment of one conductor formed into a coil around `a corresponding gatesegment of the .other conductor. Portions of the control segments `arealso used las the oonnecting wires between the sections, so that insubsequent use these connecting wires will remain superconductivethroughout the operation of the circuit in which they are incorporated.

The superconductive stock serves as a convenient material for thefabrication of all types of cryotron circuits, including elementarycryotron switches as well as the most complex circuits. Thus, as will bedescribed, a piece of stock of the fright length may lbe cut off andvarious operations may be performed on it to form a cryotron iiip-liop.It will be noted that, while the lengths of the various segments mayvary, in general, it is desirable that all the segments of eachquenching characteristic be of equal length to facilitate the formationof the stock.

In Figure 8 I have illustrated a flip-liep made from the cryotron stockof Figure 7. Six cryotrons are detached from the stock as a single unit,the cryotrons serving as K1', K2', K3', K4', K5', and K6' in Figure 8.The conductors are welded together at one end to form a connection 7similar to connection 7 and are connected through a resistor R1' to `abattery `6'. Proceeding downwardly (Figure 8) from connection 7',cryotrons K1' and K2' are connected with the control coil of each inseries with the gate of the other, as in Figure 3. At cryotron K4',however, the stock is altered by cutting, as indicated by the dottedlines 46 and 48, to isolate the control coil KA'C, 'whose terminals thenserve as read-in terminals. Gate K4'g remains in series with gate Klgtas in Figure 3. The connection between gate K3g `and control coil K'cis severed as indicated at 50, and the gate is connected in series withgate KZg -by a welded connection 52. Control ycoil K3c is isolated fromgates KS'g and K6'g, as indicated by the dotted lines 54 and 156, andcryotron K3' thus serves as the other read-in cryotron. Gate K'g,isolated at 48 and 54, and gate KSg, isolated at 56, may thus serve asread-out gates, their control coils K6'c and KSc being in series withread-in gates K4g and K3'g, respectively. The control coils lKSc and K'care also connected together at la welded junction 24' similar tojunction 24 of Figure 3.

Thus, the elements of Figure 8 -are connected in ythe sarne circuitrelationship as are their counterparts in Figure 3, and the operation ofthe two circuits is identical. However, only three internal connectionsare required in the construction of Figure 8, as compared with the tenwhich are required when the circuit is constructed from individualcryotrons.

'Ihus I have described a composite cryotron conductor having segmentswith different quenching characteristics. Segments along the sameconductor may thus serve as gate and control elements in varioussuperconductive circuits, thereby eliminating many of the weldedconnections between gate and control elements heretofore required in thefabrication of such circuits. These composite superconductive conductorsYmay be manufactured in any suitable Ifashion, and I have illustratedtwo forms which they kmay take, although others may also be foundsuitable.

I have .also described a cryotron .stock formed .from a vpair ofcomposite conductors. The superconductive stock `shown is very useful insuch cryotron applications yas flipiiops, and I have illustrated asimple hip-flop in which the number of internal welded connections isreduced from ten to three through the use of this stock. Although theabove description is specifically directed to stock comprising twocomposite conductors, it will be noted that three or more suchconductors, with segments having any desirable number of quenching fieldvalues, may be combined in a similar manner for various circuitapplications.

It will thus be seen that the objects set forth above, among those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above article and constructionswithout departing from the scope of the invention, it is intended thatall matter contained in the above description or shown in theaccompanying drawings shall be interpreted as illustrative and not in alimiting sense.

It is also to be understood that the following claims are intended tocover all of the generic and specific features of the invention hereindescribed, and all statements of the scope of the invention which, as amatter of language, might be said to fall therebetween.

I claim:

l. As an article of manufacture, a conductor for use in the manufactureof the combination of superconductive electric circuit elements andmeans for maintaining a low temperature environment therefor, saidconductor being superconductive in said environment in the absence of anapplied magnetic field and having gate portions requiring a givenmagnetic field strength to be rendered resistive and control portionsrequiring a greater magnetic field strength to be rendered resistive,said gate and control portions being periodically disposed along thelength of said conductor in end-to-end electrical superconductingrelationship.

'2. 'I'he combination defined in claim 1 in which said gate segments areof a niobium-tantalum alloy and said control segments are of niobium.

3. As an article of manufacture, a conductor for use in the manufactureof the combination of superconductive electric circuit elements andmeans for maintaining a low temperature environment therefor, saidconductor being superconductive in said environment in the absence of anapplied magnetic field and comprising a base conductor requiring a givenmagnetic field strength to be rendered resistive, said base conductorhaving formed on its periphery at spaced intervals along its 7 lengthcoatings of a material requiring a different magnetic eld strength to berendered resistive, the ends of each of said coatings being inelectrical superconductive relationship tosaid base conductor.

4. The combination dened in claim 3 in which said base conductor is oftantalum and said coatings are niobium.

5. As an article of manufacture, a conductor for use in the fabricationof the combination of superconductive electric circuit elements andmeans for maintaining a low temperature environment therefor, saidconductor being supercondnctive in said environment in the absence of anapplied magnetic field and having a set of -rst portions requiring agiven magneticield strength to be rendered resistive and a set of secondportions requiring a greater magnetic eld strength to be renderedresistive, said iirst and second portions being alternately disposedalong the length of said conductor in end-to-end superconductingrelationship, one of said sets of portions being of a material havingone of said field strengths and the other of said sets of portions beingof an alloy including said material and a material having a difIerentquenching eld strength.

6. As an article of manufacture, cryotron stock for use in thefabrication of the combination of superconductive electric circuitelements and -means for maintaining a low temperature environmenttherefor, said stock comprising a pair of conductors, each of which issuperconductive in said environment in the absence of anapplied'magnetic field, said conductors each having a central baseportion with a given magnetic eld strength and a plurality of spacedcoatings disposed along said base portion, said coatings being of amaterial having a greater magnetic quenching field strength than saidbase portion, the ends of said coatings being in electricalsuperconductive relationship with said base portion, the coated portionsof each conductor being in the form of coils wound about the uncoatedportions of the other conductor.

7. The combination defined in claim 6 in which said base portions are oftantalum and said coatings are of niobium. f

8. The combination defined in claim 6` in which said coated and uncoatedportions are periodically disposed along said conductors.

References Cited in the le of this patent UNITED STATES PATENTS Matthieset al June 13, 1939 Buck Apr. 29, 1958 OTHER REFERENCES

